Mineralogy, fluid inclusions, and isotopes of the Cihai iron deposit, eastern Tianshan, NW China: Implication for hydrothermal evolution and genesis of subvolcanic rocks-hosted skarn-type deposits
Mineralogy, fluid inclusions, and isotopes of the Cihai iron deposit, eastern Tianshan, NW China: Implication for hydrothermal evolution and genesis of subvolcanic rocks-hosted skarn-type deposits
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DOI：
10.1016/j.oregeorev.2017.01.032
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发表时间：
2017-06
影响因子：
3.3
通讯作者：
Jiahao Zheng;J. Mao;Fuquan Yang;F. Chai;Yongfeng Zhu
中科院分区：
文献类型：
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作者：
Jiahao Zheng;J. Mao;Fuquan Yang;F. Chai;Yongfeng Zhu
大多数夕卡岩矿床与侵入碳酸盐岩的花岗岩类密切相关。磁海是一个矿物组合和热液特征与世界上许多其他典型的矽卡岩型矿床相似的矿床。磁海铁矿体主要赋存于辉绿岩中，不与碳酸盐岩接触。此外，一些磁铁矿颗粒表现出不寻常的较高的二氧化钛含量。这些特征与典型的矽卡岩型铁矿不一致。人们正在积极研究不同的热液和/或岩浆作用来寻找其成因。由于缺乏系统的地质、矿物成分、流体包裹体和同位素研究，该矿床的成因类型、矿石成因和热液演化仍存在争议。夕卡岩矿物组合为辉绿岩的蚀变产物。根据岩相学观察，发现夕卡岩形成和成矿的三个主要共生阶段：进化夕卡岩阶段(石榴石-单斜辉石-浸染型磁铁矿)、逆行夕卡岩阶段(主铁矿阶段、块状磁铁矿-角闪石-绿帘石阶段和石英-硫化物阶段(quartz-calcite-pyrite-pyrrhotite-cobaltite).Overall，)。石榴石、单斜辉石和角闪石的成分与世界上典型的矽卡岩铁矿床的成分一致。在浸染型矿石中，一些磁铁矿颗粒具有较高的二氧化钛含量(>；1wt.%)，这可能是辉绿岩原岩继承而来的。在磁铁矿颗粒中观察到了一些明显的化学分带，其中核心富钛，而磁铁矿边缘则明显亏损钛。磁铁矿的结构和成分资料证实磁海铁矿床为热液成因，而不是以前认为的与富铁熔体有关。流体包裹体研究表明，进行性夕卡岩(石榴石和辉石)形成于高温(520-600℃)、中-高盐度(8.1-23.1wt.%氯化钠当量)和>；46wt.%氯化钠当量流体。早期夕卡岩破裂后，在静水条件下形成块状铁矿和逆行夕卡岩组合(角闪石-绿帘石)。这一阶段的流体具有较低的温度(220°-456℃)和盐度(8.4-16.3wt.%氯化钠当量)。石英-硫化物阶段石英和方解石中的流体包裹体也记录了相似的条件，温度范围为128°~367℃，盐度范围为0.2~22.9wt.%氯化钠当量。石榴石和石英的氢氧同位素数据表明，早期岩浆流体与外部流体(如大气水)的混合和稀释导致了夕卡岩形成后期流体温度和盐度的下降和大量铁的沉淀。铁矿中磁铁矿的δ~(18)O值在4.1~8.5‰之间变化，与其他矽卡岩型铁矿床中报道的值相似。这些值不同于世界范围内的其他铁矿，如基律纳型和岩浆型Fe-Ti-V矿床。综合这些地质、地球化学和同位素资料，证实磁海是一个与深部花岗岩类有关的辉绿岩型矽卡岩型矿床。
Most skarn deposits are closely related to granitoids that intruded into carbonate rocks. The Cihai (>100 Mt at 45% Fe) is a deposit with mineral assemblages and hydrothermal features similar to many other typical skarn deposits of the world. However, the iron orebodies of Cihai are mainly hosted within the diabase and not in contact with carbonate rocks. In addition, some magnetite grains exhibit unusual relatively high TiO2content. These features are not consistent with the typical skarn iron deposit. Different hydrothermal and/or magmatic processes are being actively investigated for its origin. Because of a lack of systematic studies of geology, mineral compositions, fluid inclusions, and isotopes, the genetic type, ore genesis, and hydrothermal evolution of this deposit are still poorly understood and remain controversial.The skarn mineral assemblages are the alteration products of diabase. Three main paragenetic stages of skarn formation and ore deposition have been recognized based on petrographic observations, which show a prograde skarn stage (garnet-clinopyroxene-disseminated magnetite), a retrograde skarn stage (main iron ore stage, massive magnetite-amphibole-epidote ± ilvaite), and a quartz-sulfide stage (quartz-calcite-pyrite-pyrrhotite-cobaltite).Overall, the compositions of garnet, clinpyroxene, and amphibole are consistent with those of typical skarn Fe deposits worldwide. In the disseminated ores, some magnetite grains exhibit relatively high TiO2content (>1 wt.%), which may be inherited from the diabase protoliths. Some distinct chemical zoning in magnetite grains were observed in this study, wherein cores are enriched in Ti, and magnetite rims show a pronounced depletion in Ti. The textural and compositional data of magnetite confirm that the Cihai Fe deposit is of hydrothermal origin, rather than associated with iron rich melts as previously suggested.Fluid inclusions study reveal that, the prograde skarn (garnet and pyroxene) formed from high temperature (520–600 °C), moderate- to high-salinity (8.1–23.1 wt.% NaCl equiv, and >46 wt.% NaCl equiv) fluids. Massive iron ore and retrograde skarn assemblages (amphibole-epidote ± ilvaite) formed under hydrostatic condition after the fracturing of early skarn. Fluids in this stage had lower temperature (220°–456 °C) and salinity (8.4–16.3 wt.% NaCl equiv). Fluid inclusions in quartz-sulfide stage quartz and calcite also record similar conditions, with temperature range from 128° to 367 °C and salinity range from 0.2 to 22.9 wt.% NaCl equiv. Oxygen and hydrogen isotopic data of garnet and quartz suggest that mixing and dilution of early magmatic fluids with external fluids (e.g., meteoric waters) caused a decrease in fluid temperature and salinity in the later stages of the skarn formation and massive iron precipitation. The δ18O values of magnetite from iron ores vary between 4.1 and 8.5‰, which are similar to values reported in other skarn Fe deposits. Such values are distinct from those of other iron ore deposits such as Kiruna-type and magmatic Fe-Ti-V deposits worldwide. Taken together, these geologic, geochemical, and isotopic data confirm that Cihai is a diabase-hosted skarn deposit related to the granitoids at depth.